This project has been designed to elucidate the mechanisms by which St. Louis Encephalitis (SLE) virus cycles in tropical ecosystems, and to compare the biological characteristics of SLE strains representing different survival mechanisms of the virus in Panama and in the U.S.A., so as to assess possibilities of periodic introduction of the virus into the U.S.A. from the neotropics with the occurrence of urban epidemics and vice versa. A close monitoring for SLE activity will be maintained at three or more distinct tropical ecosystems where the virus previously was isolated. Dynamics of transmission will be studied by virus isolation from mosquitoes with determination of vectorial capacity of mosquitoes, including transovarial transmission and host preference. Viremia patterns of suspected vertebrates will also be studied. Characterization of SLE strains will be done at Yale University.